The specific aims of this project are to test the hypothesis that baroreceptor resetting during essential hypertension is due, in part, to a morphological change in the innervation and/or blood supply to the carotid sinus baroreceptor region itself. These specific aims will be approached experimentally by determining: (1) whether hypertension precipitates changes in the blood supply to the sinus; (2) if there are morphological alterations to the sinus wall which would account for decreased distensibility of this vascular zone; (3) whether hypertension initiates or is affected by an increase or decrease in baroreceptor and sympathetic fiber innervation to the sinus; and (4) if alterations in the morphology of baroreceptor nerve terminals and the tissue they are coupled with occur before, or as a result of, hypertension. Vascular casting/scanning electron microscopy, methylene blue vital staining, selective nerve degeneration, and quantitative transmission electron microscopy techniques will be used to answer important questions about the morphology of this crucial baroreceptive zone during progressive hypertension. The spontaneously hypertensive rat (SHR) and its normotensive Wistar-Kyoto (WKY) stain will be used as a model of human essential hypertension in these studies. Little is known about the basic baroreceptor and sympathetic innervation and blood supply to the carotid sinus. The studies outlined in this application will provide much needed information, objectively obtained, about the morphological correlates of baroreceptor involvement in early and progressive essential hypertension, the etiology of which remains unknown.